Cavitation - End of My (Bleeping) Rope!

[mayorjones]

OK Guys - I'm going to reach out to you one more time and see if you can help me with a huge problem that has got me frustrated beyond anything you could imagine... This could get long, but please be patient. I'm in a fragile state

In May of this year I bought my dream boat - 2008 21' Sun Tracker Party Barge w/50 hp 2 stroke Mercury. First time out, we experienced some cavitation unless everybody was in the back of the boat (me, the wife and four kids - all relatively small). I posted some stuff here, did some research and found out it was cavitating/ventilating and that these things can be quite sensitive to weight in the boat. The motor is mounted as low as it can go, and even with the motor all the way down, unless I had 70%+ of the weight in the back, it would cavitate. Some of you suggested a new prop - it did have a three blade 12"x9 pitch. So, I ordered a 4 blade 12"x9 pitch. The first time out, with just one child and me, it was much, much better. I could actually trim it up a ways and I really felt like it had improved. Was hitting 5400 rpm out of the hole, and then when trimmed up it would drop to 5000 but all was good. That was with 1/2 a tank of gas.

So tonight, I fueled her up, took three of the four kids and the lovely bride the river and it was ABSOLUTELY HORRIBLE. Top rpm was 4,700 or so, and it cavitated like crazy. Even with the motor all of the way down and all - yes all five of us in the back and trying to cruise at half throttle it would start. Couldn't trim it up an inch. I can't remember ever being so frustrated.

So, here's where I'm at. If you're not familiar with the Sun Tracker pontoons, they've got this big transom thing that hangs off the back with a v-shaped bottom that, to me, is going to push water away from the prop, so if you're having a cavitation problem, that's not going to help. Next, is the 50 hp motor. I've heard that 3/4 of capacity is a good "minimum" motor. My maximum motor for this boat is 90 hp, so I'm not at 75% capacity. Could it be my motor is simply too small?

I've read reviews, both good and bad, about hydrofoils. Some I've read have had luck with them keeping water at the prop, especially with smaller motors. Others think they bog them down. Outside of buying a bigger motor or selling the damned thing, trying one of them seems like my final option to try unless one of you has a better idea.

Can anybody out there lean on their experiences and tell me what the heck is going on with this thing and if I'm screwed with this motor and just have to get something bigger or should I try the hydrofoil or back it off a cliff and call my insurance man? (kidding, of course).

Any input - good, bad or ugly, would be appreciated. As you can tell, I'm grasping at straws here.

Thanks guys.

Dwight

 

[tashasdaddy]

Re: Cavitation - End of My (Bleeping) Rope!

do you have a short shaft motor and a transom for a long shaft motor? sure sounds like it.

some pictures of the situation would surely help.

 

[MikDee]

Re: Cavitation - End of My (Bleeping) Rope!

I was thinking the same thing TD!

 

[lowell7963]

Re: Cavitation - End of My (Bleeping) Rope!

I can understand the frustration, but it is surely a problem that can be solved, not like a mystery intermittent motor problem.

2008 is really new. Don't know if you bought the boat from a dealer or private party. If you know the previous owner talk to them about the problem/history. Maybe that is why they sold it. Next find Sun Tracker dealer. I'm sure they will help out. Even the salses folks can look at the setup and see if that how it was originally put together, or somone else swapped the motor. It would be good to get a relationship with a good marina anyway.

I've got at 24ft Sweetwater with a 50HP Mercury Bigfoot and don't experience any of the cavitation issues or even the sensitivity to weight distribution. I don't think it is the horsepower, but he motor choice, or something else.

Hang in there you'll solve it and soon forget about all these startup prolems.

 

[actionpaintball]

Re: Cavitation - End of My (Bleeping) Rope!

You could do a simple sanity check and look at other identical similar pontoons for sale online and see what they are running for motor/mount location and shaft sizes. No reason to reinvent the wheel

 

[EGlideRider]

Re: Cavitation - End of My (Bleeping) Rope!

Tash and Action have provided some excellent ideas. After researching their suggestions, please report back.

 

[mrdancer]

Re: Cavitation - End of My (Bleeping) Rope!

Agree with others that it sounds like a short-shaft motor on a long-shaft transom. If that is the case, will the dealer correct the problem under warranty? If not, you might consider a manual jackplate - that should allow you to lower your motor about five inches and fine-tune the height. Cost will be around 2-300 bucks. You can probably find spacers that will allow you to drop your motor, or have some custom made, for a lot cheaper, but you won't be able to fine-tune.

Btw, extra cupping on the prop will help grab more water and reduce cavitation problems also.

 

[mayorjones]

Re: Cavitation - End of My (Bleeping) Rope!

Well, here's an update -

FYI - the prop on there now is cupped. 4 blade, 9 pitch, 12" cupped.

I checked with a local dealer, and they sell that boat with the Merc "Big Foot", which is a long shaft. Mine is not that version, so is likely a short shaft. Their service manager is going to do some checking and get back to me with options. Hopefully he comes up with something good.

Keep the comments/suggestions coming. They are all helpful and appreciated.

Dwight

 

[Jeep Man]

Re: Cavitation - End of My (Bleeping) Rope!

Well, here's an update -

FYI - the prop on there now is cupped. 4 blade, 9 pitch, 12" cupped.

I checked with a local dealer, and they sell that boat with the Merc "Big Foot", which is a long shaft. Mine is not that version, so is likely a short shaft. Their service manager is going to do some checking and get back to me with options. Hopefully he comes up with something good.

Keep the comments/suggestions coming. They are all helpful and appreciated.

Dwight

As I stated before, I suspect you have the wrong motor. I have a 20' Sun Tracker and it came with a 60 hp Bigfoot 4 stroke. When you said you had a 12x9 prop I knew it wasn't a Bigfoot, as the standard prop is 14x11. Bigfoot models are mostly 4 stroke but you can get a 2 stroke version. If the motor needs to be swapped out, insist on a Bigfoot. It's made for pontoons.

 

[mayorjones]

Re: Cavitation - End of My (Bleeping) Rope!

Update - I called Bass Pro Shops - where the boat came from originally. He said the motor that is on there is the same one they put all of their 21' pontoons and they've had no problems. He said it is a long shaft (based on the serial #). They checked and the 12x9x3 blade prop that it did have (prior to me changing it to a 12x9x4 blade) was the exact prop that Tracker suggested using. Neither prop has worked for me. Hmm'...

So then I called the guy that had the boat before me (got his name off the title). He was VERY cordial but claimed he never had this problem even once, and he had 13 people on there at one time. He had no reason to lie to me, and I believed him 100%. I asked if he had used it on a river or lake, and he said both. I use it solely on the Mississippi River, which does have a lot of current, but it will do it either direction (upstream or downstream).

So, something has happened. Either the dealer (who sold it on consignment for the original owner) swapped it out for a short shaft without telling anyone, which I don't think is the case, or something else is going on. I called a local Mercury dealer today and he told me how to measure to determine long shaft vs short shaft. If it is indeed a long shaft, I am at a loss as to what to try next. They want $5000 to trade up to a 90 hp, and I'm not in a situation that I care to spend another $5k after just spending $15k on the boat.

Perplexed in Iowa -

Thanks for listening.

Dwight

 

[79monarch]

Re: Cavitation - End of My (Bleeping) Rope!

Got any Pics?

 

[Bifflefan]

Re: Cavitation - End of My (Bleeping) Rope!

do you have a short shaft motor and a transom for a long shaft motor? sure sounds like it.

some pictures of the situation would surely help.

I agree,, Sounds like a short shaft problem...

 

[steelespike]

Re: Cavitation - End of My (Bleeping) Rope!

First are you sure you aren't picking up junk on the prop.
Sure seems something is different with your boat.You might look underneath.
It appears when loaded up something underneath is disturbing the water.
Steering/throttle cables.A loose or bent brace.Distorted splash shield.Something added that is disturbing the water.You might measre top of transom to bottom of tubes.Then compare to other tracker toons. Or for that matter any toon of similar size.

 

[mayorjones]

Re: Cavitation - End of My (Bleeping) Rope!

Went and checked tonight - the part number on the motor is 50ELPTO. I believe that the "L" in here means "Long"? If so, I think we can rule out the short problem, right? If so, I am truly at a loss. I will take pics tomorrow, but it looks crystal clean underneath - the thing is truly like brand new.

I'll post pics tomorrow, but for now, the mystery continues....

Thanks.
Dwight

 

[rickdb1boat]

Re: Cavitation - End of My (Bleeping) Rope!

I think smaller prop on the "Non-Bigfoot" motor is the main issue. You seldom hear of this happening with the bigger HP motors that use larger gearcases and swing the bigger props (both in diameter and blade surface). And to compound the problem, that motor weighs less than half of what the maximum rating is for that boat (115 HP) and that extra 200 lbs from the larger engine would naturally put the prop deeper in the water. In all reality, the dealer should have never sold you a standard motor on a pontoon. The newer toons also have larger logs (26" in this case) and sit high out of the water. Probably the only way to cure it is by going to an extra long shaft motor or a Bigfoot. You could try a hydrofoil, but if you use the type that requires drilling into the lower unit to install it, it may void the waranty. Just from your description of "it gets better when everyone moves to the back" tells me the motor is too high. I'd be asking the dealer to make me a very generous deal on a Bigfoot...

 

[Silvertip]

Re: Cavitation - End of My (Bleeping) Rope!

This is what you said in an earlier post: Some of you suggested a new prop - it did have a three blade 12"x9 pitch. So, I ordered a 4 blade 12"x9 pitch. The first time out, with just one child and me, it was much, much better. I could actually trim it up a ways and I really felt like it had improved. Was hitting 5400 rpm out of the hole, and then when trimmed up it would drop to 5000 but all was good. That was with 1/2 a tank of gas.

So tonight, I fueled her up, took three of the four kids and the lovely bride the river and it was ABSOLUTELY HORRIBLE. Top rpm was 4,700 or so, and it cavitated like crazy. Even with the motor all of the way down and all - yes all five of us in the back and trying to cruise at half throttle it would start. Couldn't trim it up an inch. I can't remember ever being so frustrated.
------------------------------------------------
Your RPM numbers do not make any sense at all because an engine should rev higher as you trim up rather than down as you indicate. Next, dropping from 5400 to 4700 or so is also not a realistic observation. In fact, with a 12x9 prop the rpm on your engine may very well have been correct at 4700. The solution to your problem is change the engine. Whoever configured that boat should be flogged. A standard ELPTO Merc has a 1.83:1 lower unit gear ratio which DEMANDS a small diameter prop because of the resulting prop speed (not boat speed). At very low pitch numbers it will simply dig a hole in the water rather than providing any reasonable push. Big Foot motors carry a 2.33:1 gear set and for pontoon service run a 14 x 11 elephant ear four blade prop specifically designed for pontoon service. The Big Foot leg is really the same leg as the larger 75 and 90 HP engines. So stop spending money on props and get the right engine. You will see a ton of 40 and 50 HP Johnson/Evinrude engines on your size pontoon. They work well because they have a 2.34:1 gearcase and can swing higher pitch pitch and 12-1/2 inch props very efficiently. I'm not suggesting you switch to a J/E engine, just using them as a comparison. In summary, your engine is not set up to run anything other than the 10-1/2 inch diameter props because of the gear ratio and that alone makes it unsuitable for pontoon service.

 

[lowell7963]

Re: Cavitation - End of My (Bleeping) Rope!

Ok My 2 cents worth again. You've done all the right steps, talking to the previous owner, dealer etc. Now I'd contact Suntracker. I'm sure they don't want a bad rap for their boats, and they can certainly clear up what motor they would recommend, and they would like to know if dealers are selling their product and putting on inappropriate motors. I'm sure they want satisfied customers, even the second or third owners.

It is not directly solving your problem, but you may find they are helpful in resolving it, maybe some deal on getting the proper motor on. Like you say upgrading to a 90HP (actually a 50HP Bigfoot would probably brighten your day) is very expensive.

Good Luck

 

[sumcat1]

Re: Cavitation - End of My (Bleeping) Rope!

I had a similar problem with a 20' Parti-Kraft. It came with a 50hp Johnson and a 9 pitch prop. The manufacturer (Godfrey) sent me a new transom(warranty) which I installed myself. That was a partial fix. Ended up with a 13 pitch prop which took care of the rest. The dealer can have your 9 pitch rebuilt to a 13 pitch for less than new. Thats what I did. Try that before you spend big bucks on an engine swap. Someone suggested a manual jackplate and that may work but you may have to mount it upside down to drop the engine enough. Definitly stay away from a hydrofoil. I now have a Suntracker 21' fishing barge with 60hp Bigfoot. No problems. Also be sure your tach is set right. I couldn't seem to reach max RPM and found out my tach was set for the wrong engine config.

 

[Silvertip]

Re: Cavitation - End of My (Bleeping) Rope!

Wrong info. A 13P prop on a 1.83:1 gearcase is not an acceptable solution. A 13P prop is the standard prop for a 15 - 16 foot planing hull - not a pontoon.

 

[mayorjones]

Re: Cavitation - End of My (Bleeping) Rope! - Update with Pics!

Re: Cavitation - End of My (Bleeping) Rope! - Update with Pics!

Thanks everybody for your responses - I'm getting "Buy a different motor" and "Don't buy a different motor yet" messages - still trying to determine best route to fix the problem before summer is toast.

Attached are some pics for your review - showing how the motor sits, gear ratio, mounting, etc... Hoping someone sees something that might look suspicious (though in the end it may simply be the wrong motor, though it is SO weird that the guy at Bass Pro would tell me this is the same motor they use on all of them.)